It’s Friday so let me continue to walk along the details of Mahan’s profile:
The glacis is a small mound of earth raised in front of the ditch ; it is seldom used in field works, and is therefore not a constituent part of their profile.
Tag this as “optional.” So if you were looking to cut down on the amount of dirt movements, you can skip the glacis.
Well not so fast. Why have it in the first place? Recall what I said earlier about the relation between the angle of the Superior Slope and the width of the Ditch. Mahan wrote:
The superior slope is arranged to defend the crest of the counterscarp ; to effect which the fire should not strike below the crest, nor pass more than three feet over it ; otherwise, either the counterscarp would be damaged, or the assailed by stooping when near the crest would find a shelter. The inclination of the superior slope, however, should not be greater than one-fourth, nor less than one-sixth, that is, the base of the slope she uld be between four and six times the height. If greater than one-sixth, it would make the portion of the parapet, about the interior crest, too weak ; and if less than one-sixth, the ground directly in front of the work would not be so well defended ; more-over, as artillery cannot be fired at a greater depression than one-sixth, without injuring the carriage, this inclination of the superior slope serves as a check in rapid firing.
So we need to go back to the line I offered last Friday:
This depicts the desired angle, relative to points E (the Exterior Crest on the Parapet) and either point K or M (the crest of either the Counterscarp, if needed, the Glacis, respectively). This was a critical measure for the fortification. Let us dissect the profile a bit:
What Mahan said is that if bullets fired on a line from behind the Parapet would strike at a point beyond outer crest of the ditch, then it offered an uncovered space in front of the works. Consider that a “shallow” angle between E and K. That is depicted with the red line on the diagram above. Such was a serious flaw the defender wished to avoid. However, if the line of fire offered a steep angle, that implied the defender had to raise up too much over the parapet (the blue line on the diagram). And thus the Banquette was too high and needlessly exposed the defenders. Oh, and also any artillery place on the Banquette might not depress to engage the enemy. The “just right” zone for this line offered an angle in which bullets fell directly on the crest of the Counterscarp, or just three feet below it. That ensured the enemy was not granted a place to gain a foothold, short of the Ditch. Furthermore, while in the Ditch the enemy could not engage the men on the Parapet. If set “just right” the Ditch would become a place where the enemy could only pause to contemplate just how precarious the situation was (and perhaps start waving white flags). But nature rarely offers the engineer a “just right” solution. So often there was a need to adjust nature to achieve that “just right” angle. And the Glacis was that adjustment:
If, owing to the command, the fire should pass higher than three feet above the crest of the counterscarp, it would then be necessary to construct a glacis in front of the ditch. It must be so arranged that it can be swept by the fire of the work, and be commanded by it at least five feet.
Mahan defined that as the lines between the Crest of the Counterscarp (K), Crest of the Glacis (M), and Foot of the Glacis (N). (Yes… no L, as it looked too much like an I).
There was no fancy names offered for these lines. Let me, following some convention, arbirtrarily call the line between K and M (I’ll explain that K’ in a second) as the Interior Slope of the Glacis:
Clearly this had support the crest (M) at a height to achieve that “just right” angle. But, as with the discussions of the slopes of the parapet and ditch, the desired angle of the slope was no steeper than that at which earth normally piles. Otherwise it became a maintenance issue.
I’ll call the line from the Crest of the Glacis to the Foot of the Glacis as simply the Exterior Slope of the Glacis. Feel free to come back with a more proper term, if you wish.
The Exterior Slope, in keeping with the function of the Glacis, would lay on the same line as that from the Parapet’s Superior Slope.
But there was one other line to consider here, and where K’ comes in.
This is not labeled on Mahan’s diagram, it’s my convention. But Mahan did discuss this line. Just as with the Parapet and Ditch, were a Berm was often needed to off-load the weight of the piled earth, the Glacis often had to be placed away from the Ditch to prevent a maintenance problem. So this was in effect another berm.
But this line of K to K’ had another desired purpose. If desired for the defense, it could become an advanced Banquette of sorts. Defenders on that section could delay, if not stop, attackers attempting to scale the Glacis. Furthermore, the protection afforded by the Glacis would allow defenders to maneuver on the exterior of Ditch if needed. Taking a term from more elaborate fixed fortifications, this was known as the Covered Way.
Overall, we might easily rate the Glacis as simply an option for those who had time and labor to expend making a work just a bit more formidable. However, we must consider those angles – shallow and steep – between the Parapet and Ditch. A well constructed Parapet might be undone by the lack of a Glacis.
(Citations from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, pages 3 and 21.)